1. Field of the Invention
This invention relates generally to a method for eliminating defects from semiconductor material, and more specifically to the cryogenic treatment of semiconductor materials, e.g. “Indium-Phosphide” (InP), “Gallium-Arsenide” (GaAs), “Gallium-Nitride” (GaN), and “Gallium-Phosphide” (GaP), semiconductor ingots, semiconductor substrate wafers, and epitaxially grown semiconductor layers and other structures as the means to provide for the elimination of dislocation and other defects from said material and structures.
2. Description of Related Art
The invention generally relates to processes currently used to reduce the number of defects that form in large single crystals, and more particularly to processes used to grow defect free large single crystals of Group III-V semiconductor compounds. One of the most significant developments in recent years in semiconductor technology has been the increased importance of compound semiconductors. Particularly significant are the Group III-V semiconductor compounds composed of elements of Groups III and V of the periodic table, such as GaAs and InP. Compound semiconductors, such as InP are currently used in devices such as semiconductor “Laser Diodes” (LDs), “Light Emitting Diodes” (LEDs), microwave oscillators and amplifiers, high-speed transistors, e.g. “High Electron Mobility Transistors” (HEMT), and various types of radiation detectors including infrared and visible light detectors.
Because a higher electron drift velocity makes possible faster switching speeds over the more conventional devices constructed using silicon, GaAs is increasingly being used for integrated memory and logic circuitries. The commercial use of semiconductors requires the growth of large single crystals of defect free semiconductor material.
Various methods have been proposed for growing large single crystal ingots from which defect free substrate wafers can be cut for the subsequent fabrication of useful electronic and electro-optic devices. One of the more promising methods for such crystal growth is the “Vertical Gradient Freeze” (VGF) method, particularly the VGF method defined in the U.S. patent of W. A. Gault U.S. Pat. No. 4,404,172. According to this method, polycrystalline starting construction material is placed in a vertically extending crucible including a small cylindrical seed-well portion at its bottom end, which snugly contains a seed-crystal. Initially, the starting material and a portion of the seed are melted. The power to the system is then reduced in such a manner that freezing proceeds vertically upwardly from the seed-crystal.
The major advantage of the VGF method over more conventional methods, such as the Czochralski growth method, is that morphous single crystal melts with very ‘low dislocation densities’ (i.e., a reduction in defect density) can be produced using low thermal gradients and slow rates of cooling. However, because the interaction of the melt with the crucible normally leads to the introduction of dislocations that cause false grains to nucleate and spoil portions of the single crystal the VGF method has its limitations.
Further, it is well known that the III-V compounds tend to dissociate at higher temperatures, with the more volatile Group V elements escaping into the vapor phase. Several approaches have been developed to prevent or retard this tendency. For example, in one approach to the growth of GaAs crystals, the more volatile Arsenic component is prevented from escaping by providing a vapor pressure of Arsenic vapor over the melt from a separately heated reservoir of Arsenic within the sealed growth container.
Moreover, it is also known in the art that Arsenic loss from the melt may be retarded with the use of any of various materials such as “Boric-Oxide” (B2O3), “Barium-Chloride” (BaCl2), or “Calcium-Chloride” (CaCl2), which act as diffusion barriers. Such additives, having a lower density than the molten GaAs, rise to the surface, encapsulate the melt, and together with an inert gas pressure in the vessel, can contain the volatile Arsenic vapors; e.g., see the paper “Growth of Single Crystals of GaAs in Bulk and Thin Film Form,” by B. A. Joyce, included in the book “Crystal Growth,” edited by B. R. Pamplin, Pergamon Press, 1975, pp. 157-184 at page 165.
Furthermore, various attempts have been made over the years to grow III-V crystals by the VGF technique utilizing different crucible materials. For example, see the aforementioned Pamplin book at pp. 389-391. A significant limitation on the success of these efforts has been the physical and chemical interaction of the melt with the crucible wall. For example, see the aforementioned Pamplin book, at p. 389, and “The Art and Science of Growing Crystals,” by J. J. Gilman, Ed., John Wiley & Sons, New York, 1963, at p. 366 and p. 390.
FIG. 1 illustrates an apparatus that has been used for the selective growth of the boric oxide layer in accordance with the prior-art. Crucible 11 is contained within a cylinder 10, which in turn is contained within a furnace 13. As the furnace heats the crucible, oxygen (O2) is introduced into the interior of the crucible by a tubulation 14. Simultaneously, a counterflow of inert gas, such as nitrogen (N2), is introduced into cylinder 10. The nitrogen thus enters the open end of the seed-well portion of the crucible 11 so as to pre-vent oxygen from entering from the opposite end. Thus, both oxygen and nitrogen are expelled at the same end of the cylinder 10 as shown by the arrows that designate gas flow.
By simple experimentation, one can quickly obtain a placement of tubulation 14 that, with an appropriate flow of nitrogen, gives thermal oxidation of the frusto-conical transition portion without oxidizing the inner surface of the seed-well portion of the crucible 11. The flow of nitrogen in cylinder 10 along the outer surface of crucible 11 prevents oxide growth on the outer surface of the crucible. For growth of a boric oxide layer 15-μm thick, the crucible was heated at 1050° Celcius for 18 hours, while being exposed to O2 flowing at a rate of 40 liters per hour (1/hr).
During growth of layer 12, the structures are preferably arranged in an axially symmetric environment (FIG. 2), to enhance uniformity. Nevertheless, it has been found that eddy currents of nitrogen sometimes occur around the large open end of the major portion of the crucible so as to reduce the thickness of the layer 12 at that end. It has proved difficult to reproducibly decouple this interaction in order to eliminate spurious polycrystalline growth. In conclusion, while the VGF method has succeeded in eliminating some dislocation defects in III-V comprised crystals, as described above, it clearly has its limitations—the most significant being higher manufacturing costs.